Television transmitter



Jan. 31, 1939. H owsE-r'r 2,145,295

TELEVISION TRANSMITTER Filed April 15, 1936 INVENTOR H. M. DOWSETT AND L. E.Q. WALKER ATTORNEY Patented Jan. 31, 1939 UNITED STATES PATENT OFFICE TELEVISION TRAN SMITTER Application April 15,

1936, Serial No. 74,398

In Great Britain April 18, 1935 4 Claims.

This invention relates to television and like transmitting apparatus. One feature of the invention provides improved means for translating images of a subject of transmission into electrical signals, said means being of the non-mechanical type, i. e. of the type not requiring mechanically moving parts for scanning action.

A second feature of the invention provides improved means whereby in addition to signals corresponding to light and shade values in the individual elemental areas of a subject of transmission, there may be obtained signals corresponding to the general illumination level or background brilliance of the subject. As will be seen later the two features of this invention may be, and preferably are, employed in combination, but nevertheless the second feature is not strictly limited to its application to transmitters in accordance with the first feature.

According to the first feature of this invention, a television or like transmitter comprises a fluorescent screen, means for scanning said screen with a spot obtained by the incidence thereon of a sharply focused cathode ray beam, a photosensitive cathode, means for projecting images of the subject for transmission upon said cathode, an anode associated with said cathode, and means for subjecting said cathode to a scanning spot of additional light obtained by reason of the incidence of the cathode ray beam upon the fluorescent screen the intensity of the images projected upon said photo-sensitive cathode, the intensity of the scanning light spot and the whole arrangement being such that said cathode is operated over a non-linear portion of its photoelectric current (ordinates) activating light intensity (abscissae) characteristic whereby the additional photo-electric current caused by the additional activation by the scanning light spot (derived from the fluorescent screen) of individual areas of said photo-sensitive cathode is caused to vary with the initial activation of said areas by the image light thereon. Preferably the fluorescent screen and the photo-sensitive layers are constituted by depositions on opposite faces of a sheet of light permeable material arranged transversely in an envelope which acts as the envelope both of the cathode ray tube and photocell portions of the apparatus.

According to the second feature of this invention there is employed, in combination with a device for translating images of a subject of transmission into electrical signals and which is such as to give an electrical output composed of two components as above set forth, a picture signal amplifier whose gain is arranged to be automatically controlled in dependence upon the magnitude of the second mentioned of the two said components.

The invention is illustra din the accompanying drawing in which Figure 1 shows diagrammatically and schematically an embodiment of both features of the invention, Figure 2 being an explanatory graph.

Referring to Figure 1 a television or like transmitter comprises a cathode ray tube I having at one end the usual electron gun and cathode ray beam focussing system, and co-ordinate deflecting means, e. g. mutually perpendicular pairs of deflecting plates. In Figure 1, 2 represents the cathode, 3 the control electrode, 4 the accelerating electrode, and 5 and 6 mutually perpendicular pairs of electrostatic deflector plates. Towards the other end of the tube is a transverse glass partition represented by the full line I which is prepared upon the side facing the electron gun with a suitable fluorescent material represented by the dotted line 8. Upon its other side the glass partition I is provided with a photo-sensitive layer represented by the dotted line 9 and on the side of said layer remote from the electron gun is an anode consisting, for example, of a small conductive ball l0 carried upon a suitable support ll. Optical images of the subject for transmission e. g. images of a scene or obtained from a cinematograph film (in the case of a telecinematograph transmitter) are focussed upon the photo-sensitized side 9 of the glass partition I. The cathode ray beam (represented in one of its possible positions by the chain line I2) is focussed to as flne a spot as possible upon the fluorescent layer 8 on the glass partition 1. ,Suitable saw-tooth current or potential waves from a source (not shown) are applied to the deflecting means 5, 6, so as to cause the cathode ray beam to scan a predetermined area on the fluorescent surface 8 this area corresponding with the image area which is upon the other side of the glass partition. The anode l0 collects all the photoelectric emission from the photo-sensitive layer.

It will be seen that with this arrangement the photo-electric current to the anode ill will at any instant include a component due to activation of a spot on the photo-sensitive layer by light occasioned by the incidence of the cathode ray beam upon the fluorescent layer 8, and, of course, as the cathode ray beam scans the fluorescent layer the particular spot upon the photo-sensitive surface which is additionally illuminated by reason of the incidence of the cathode ray beam upon the fluorescent layer will change. The photo-sensitive cathode is such and the illumination thereof is so chosen that the photo-electric cell constituted by the said cathode and its co-operating anode, operates over a nonrectilinear portion of its current (ordinates) light flux (abscissae) characteristic curve. Sucha characteristic curve is shown in Figure 2 where the ordinate values are values of photo-current and the abscissae values are values of activating light. As will be apparent from Figure 2 the anode current of the photo-electric cell will vary in accordance with the light and shade in the individual portions of the images projected 'upon the photo-sensitive cathode forthe reason that, since the cell operates over a non-rectilinear portion of its characteristic, the increase in photo-, electric omission from the spot additionally illuminated by the cathode ray beam at any instant will not be a constant but will depend upon what may be termed the image illumination of that spot.

. It will now be appreciated that, in a transmitter in accordance with the first feature of this invention and as so far described with reference to Figure 1 the anode current will in effect comprise two components (I) that due to the light and shade values in the elemental areas of the subject of transmission and (2) that due to the general illumination or background brilliance of the said subject.

In one way of applying the second feature of this invention to a transmitter in accordance with the first feature of the said invention and as so' far described with reference to Figure 1, the anode ll) of the photo-electric cell portion of the transmitter tube is connected to the cathode i3 of a thermionic amplifier valve ll of the socalled variable ,i type and is also connected through a resistance IE to the positive terminal of a potential source i6 whose'negative terminal is connected to the photo-sensitive cathode 9. The junction point of this resistance and potential source is connected through a negative bias potential source I! to the control grid [8 of the variable t valve l4 and output signals are taken from the anode circuit thereof. With this arrangement current components corresponding to the elemental light and shade values in the subject of transmission set up voltages across the resistance l5 so that picture point signals are applied to the grid l8 but, in addition, the grid bias on the valve ll will vary in accordance with component currents due to general illumination or background brilliance, for when this is high the average current through the resistance i5 will be increased and the net negative bias on the valve will be decreased. Accordingly the output signals from the valve will be increased in amplitude when the background brilliance is high and vice versa.

Having now particularly described and ascertained the nature of our said invention and in what manner the sameis to be performed we declare that what we claim is:-

1. In a'television transmitter, a cathode ray tube having a luminescent screen positioned substantially in one end thereof, means within the tube and located at the opposite end of said tube from said luminescent screen for developing an electron beam and for directing the said beam upon said luminescent screen, means for deflecting said beam to cause it to traverse in a twodimensional pattern of the luminescent screen to produce, upon impact, light over an area' of the luminescent screen commensurate in size with the cross-sectional area of the developed electron beam, a light permeable partition positioned within-said tube and adjacent the luminescent screen, said partition being located on ,the side of said screen remote from said electron beam developing means, a photoelectrically responsive coating covering the said partition and located on the opposite side of said partition from said luminescent screen, said photoelectric coating being adapted to be illuminated by the light of an optical image, said photoelectric coating having a non-linear response with increasing light values projected thereupon, a pin-like anode element positioned within the tube intermediate the photoelectric coating and the source of the optical image to receive the photoelectrons released from the photoelectric coating under light activation, connections for applying potentials to said photoelectric coating and anode and an amplifier having its input circuit connected with said connections to said photoelectric coating and the pinlike anode and arranged to have its output energy controlled in accordance with the photoelectric current flowing to said anode.

2. In a television system a cathode ray tube, a light permeable partition dividing the cathode I ray tube into two separate sections, a luminescent coating supported upon one surface of the light permeable partition and a photoelectric coating supported upon the opposite surface of said light permeable partition, a pin-like anode element positioned in the section of the tube including the photoelectric coating to receive the electrons released from the photoelectric coating by light activation, means for applying potentials to the photoelectric coating and to the pin-like anode member, a load resistor connected between the photoelectric coating and the anode member and an amplifier connected across the load resistor, electron gun means positioned in the section of the tube including the luminescent coating for developing a cathode ray beam and projecting said developed beam upon the luminescent coating to produce a luminous spot to increase by a fixed increment the photoelectric emission from the photoelectric coating upon the opposite side of the light permeable partition from the luminescent coating, and means for moving the electron beam according to bi-directional patterns over 'the luminescent coating to cause variations in voltage drops across the load resistor to modulate said amplifier.

3. The system claimed in claim 1 comprising, in addition, means for electrically controlling the gain of said amplifier in accordance with the average photoelectric emission of the photoelectric coating.

4. The system claimed in claim 1 comprising, in addition, a direct connection between the input of said amplifier and said pin-like anode and photoelectric coating for automatically controlling the gain of said amplifier in dependence upon over components of said picture signals which corresponds to the background brilliance of the subject of which the electro-optical image representation is to be produced.

HARRY MELVILLE DOWSETT.

LOUIS EDWARD QUINI'RELL WALKER. 

